Our solar system contains two distinct groups of planets, referred to as the inner and the outer planets respectively.
The inner planets, which are closer to the Sun, consist mainly of rock and are thus similar to the Earth. They are Mercury, Venus, the Earth and Mars. They are all relatively small planets and share several characteristics. For instance, they all have a solid surface and are made up of similar materials, mainly rock and iron.
Their diameters lie between 12,756 km (Earth) and 4880 km (Mercury). The orbits on which they move are mainly circular. Those of Mars and Mercury are, however, more eccentric than those of the other two.
The orbits of Mercury and Venus lie within that of the Earth. Both planets can be observed to have phases identical to those of the Earth moon, moving from "new" to "full". Neither have moons.
Mercury has a particularly high density, which is thought to be caused by a particularly large amount of iron in its core. Its atmosphere is thin. Regions with many craters can be observed on its surface.
Venus is shrouded in dense layers of cloud. Its atmosphere consists of 97% carbon dioxide, causing a greenhouse_effect with temperatures around 500┬░ Celsius.
The Earth is the only planet that possesses an abundance of liquid water and it supports life. It is accompanied by a natural satellite, the Moon. Mars has two moons called Deimos and Phobos. They have few similarities with the Earth's moon. It is believed that, at some stage, there was water on Mars. Today, the planet is cold and barren. Its poles are covered in dry ice.
